1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a fixing belt or a fixing film to apply heat to the recording medium bearing the toner image. FIG. 1 is a sectional view of a typical fixing device 20R1 including such fixing belt 204. The fixing belt 204 is looped around a heating roller 202 and a fixing roller 203, and a tension roller 206 biases the fixing belt 204. A pressing roller 205 presses against the fixing roller 203 via the fixing belt 204 to form a nip N between the pressing roller 205 and the fixing belt 204. The fixing belt 204 is heated by a heater 201 provided inside the heating roller 202. As a recording medium P bearing a toner image passes between the fixing roller 203 and the pressing roller 205 on the fixing belt 204, the fixing belt 204 and the pressing roller 205 apply heat and pressure to the recording medium P bearing the toner image to fix the toner image on the recording medium P.
One problem with such an arrangement, however, is that the heating roller 202 has a relatively large heat capacity, resulting in a longer warm-up time for the fixing device 20R1. To address this problem, instead of the fixing belt 204 the fixing device may employ a fixing film having a relatively small heat capacity. FIG. 2 is a sectional view of a fixing device 20R2 including such fixing film 213. A ceramic heater 211 is provided inside a loop formed by the fixing film 213. A pressing roller 212 presses against the ceramic heater 211 via the fixing film 213 to form a nip N between the pressing roller 212 and the fixing film 213. As a recording medium bearing a toner image passes between the pressing roller 212 and the fixing film 213, the fixing film 213 heated by the ceramic heater 211 and the pressing roller 212 apply heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium.
However, the fixing film 213 also has a drawback in that, over time, friction between the ceramic heater 211 and the fixing film 213 sliding over the ceramic heater 211 increases, resulting eventually in unstable movement of the fixing film 213 and increasing the required driving torque of the fixing device 20R2. Moreover, the fixing film 213 has another drawback in that the ceramic heater 211 heats the fixing film 213 at the nip N only and therefore the rotating fixing film 213 is coolest when it reenters the nip N, resulting in formation of a faulty toner image on the recording medium due to the low temperature of the fixing film 213.
To overcome these drawbacks, instead of the ceramic heater 211 the fixing device may employ a heat generator provided inside the loop formed by the fixing film to heat a part of the fixing film. The heat generator is supported by a heat generator support provided inside the loop formed by the fixing film and generates heat to be transmitted to the fixing film. As the fixing film rotates while supported by a fixing film support provided inside the loop formed by the fixing film, the fixing film is heated by the heat generator.
A slight gap of predetermined size is provided between the fixing film and the heat generator and between the fixing film and the fixing film support. However, the heat generator support and the fixing film support may be deformed by the heat generated by the heat generator, varying the gap provided between the fixing film and the heat generator and between the fixing film and fixing film support. Accordingly, heat generated by the heat generator may not be transmitted to the fixing film uniformly in the axial direction of the fixing film due to the varied gap. Consequently, a part of the heat generator may be overheated due to insufficient heat transmission therefrom to the fixing film, resulting in malfunction of the fixing device.